The processing of semiconductive wafers (e.g., thin discs of single-crystal silicon) into various integrated circuits (and similar devices) is well known in the art. To this end manufacturers offer throughout the world various makes and designs of equipment for this purpose. Because of the precision in construction and of operation required of such equipment, and the uniformity in processing necessary to obtain a high yield within specifications of devices being produced, the equipment is expensive to build and to operate. It is highly desirable therefore that the capital and operating costs of such equipment, for a given production throughput, be reduced as much as possible.
Recently semiconductive wafers with a diameter of 300 mm (0.3 meter) have become available to the manufacturers of integrated circuits. Compared to previously available 200 mm wafers (or even smaller ones), a 300 mm wafer offers a potential gain in productivity of more than two to one. Use of 300 mm wafers is thus highly attractive from a cost standpoint.
In a SACVD.TM. process step where silicon oxide is being deposited as insulation on a wafer, reactive gasses (well known in the art such as an organic vapor in helium or nitrogen, and ozone) are separately mixed together very close to where they will be used, then immediately introduced into a hermetically sealed chamber. The mixed gasses flow into a chamber at desired pressure and flow rate and are continuously exhausted from the chamber by a pump. A wafer within the chamber is held at a desired temperature (e.g., in the range of 200 to 800.degree. C.) while the reactive gasses flow over an exposed surface of the wafer and in so doing deposit thereon a thin layer of silicon oxide insulation. Since the layer of silicon oxide being deposited onto the wafer should be as uniform as possible over the entire wafer surface from center to rim, the reactive gas stream should have its component gasses thoroughly mixed together before impinging on the wafer, and the mixed gasses should flow with perfect, or near perfect, uniformity over the entire area of the exposed surface of the wafer.
Non-uniformity in mixing and/or flow of the reactive gasses results in an insulating layer (SiO.sub.2) being deposited unevenly onto the wafer. The resulting layer is thus thicker, or thinner, in some places than in others. When even small peaks and/or valleys begin to show up in an insulating layer the integrated circuits (or similar devices) which are being produced on the wafer can be rendered defective and thus become scrap. It becomes however, more and more difficult to achieve absolute uniformity in the mixing and flowing of larger volumes of the reactive gasses as the area of a wafer is made larger and larger (e.g., from a diameter of 200 mm to a diameter of 300 mm or greater). Thus, in practical effect, processing apparatus intended for 200 mm wafers cannot merely be scaled up in size so that it is big enough to handle 300 mm wafers and still produce integrated circuits having zero, or nearly zero defects. Substantial modifications in the apparatus are required. The present invention in one of its aspects provides an effective and economical solution to this problem of achieving uniform processing in chambers for large diameter wafer (e.g., 300 mm).
Previously, where wafer diameters were much smaller, there have been attempts to combine two wafer-processing chamber cavites into a single piece of equipment. Thus common usage could be made of certain elements of equipment such as housing, platform, gas supplies, control circuits, etc. The provision of dual-cavity chamber equipment would therefore offer increased production throughput along with substantial savings in capital cost. But problems of uniform processing, as discussed above have, among other reasons, precluded dual-cavity chamber equipment suitable for 300 mm wafers. The present invention in another of its aspects makes possible dual-cavity chamber apparatus capable of processing two such semiconductor wafers simultaneously.